Apparatus for forming cathodes



fh... L Jr Jan. 20, 1959 K. L. LINDSAY 2,870,315

APPARATUS FOR FORMING CATHODES Filed Jan. 18, 1954 JNVENTOR. AE/v/vfr/fl L WD5/Wi BY ffm,

United States Patent() APPARATUS non roaMiNG cATnoDEs Kenneth L. Lindsay, Van Wert, Ohio, assigner to International Telephone and Telegraph Corporation Application January 18, 1954, Serial No. 404,699

1 Claim. (Cl. 219--35) The present invention relates to a method and apparatus for forming radiant energy cathodes, and more particularly to a method and apparatus for forming a cathode on the face plate of an evacuated, cylindrically shaped glass envelope.

ln the formation of silver-oxide-caesium cathodes which emit electrons in response to radiant energy projected thereon, the usual process incorporates the step of releasing pure caesium inside an evacuated bulb or envelope which is provided with a base or support upon which the cathode is formed. in the process of liberating the pure caesium, the caesium vapor tends to distribute itself throughout the bulb and to react with various portions thereof as temperature and other conditions may allow. By the use of certain materials, such as silver oxide, as a base for the cathode, the caesium may be caused to condense thereon and to react therewith to provide a film which is sensitive to radiant energy. In order to control the reaction of the caesiurn with the silver-oxide, the entire tube is maintained within critical temperature limits, whereupon the major portion of the released caesium may be caused to deposit in the desired area only constituting the cathode surface. However, since the caesium has a great attinity for oxygen and furthermore is a highly reactive metal, it will tend to deposit and react with the remaining portions of the tube envelope and with the metallic electrodes contained in the envelope. This caesium reaction with the other portions of the tube is undesired since these portions become sensitive to radiant energy and liberate electrons which interfere with the operating characteristics of the tube.

Therefore, it is an object of this invention, to provide a method and apparatus for forming radiant energy cathodes. which will tend `to prevent the reaction of the caesium material with portions of the tube other than the cathode surface.

It is another object of this invention to provide a method and apparatus for conlining the deposition of caesium to a limited area on the inside wall of an evacuated tube. Y

It is a Astill further object of this invention to provide an apparatus for forming silver-oxide-caesium cathodes on the face plate only of a cylindrically shaped evacuated tube containing metallic electrodes by the use of at least two separate heat sources which maintain the face plate of the tube at a different temperature from that of the remaining portions of the tube which include the envelope and the metallic electrodes.

It is yet another obiect of this invention to provide an apparatus for forming radiant energy-sensitive cathodes on the face plate of a completely assembled vacuum tubs which comprises two separate heating sources which are so controlled as to maintain the temperature of the face plate constant and at a level which differs from the temperature of the remaining portions of the tube assembly.

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Other objects will become apparent as the description proceeds.

In accordance with the present invention there is provided an oven having two heating chambers which areequipped respectively with radiant heat-generating elements and which are fully enclosed with the exception of a tube-receiving opening in a baille separating the two chambers. The chambers and the heating elements are so arranged that with a tube positioned inside the opening, the opposite ends of the tube will be maintained at different respective temperatures. By means of this differential heating of the tube, deposition of the caesium on the tube face plate is facilitated and unwanted deposition on other parts of the tube assembly is substantially reduced.

For a better understanding of this invention, reference is made to the following description, taken in connection with the accompanying drawings, with the scope being pointed out in the appended claims.

In the accompanying drawings:

Figure l is a longitudinal section of a radiant energysensitive vacuum tube which may be provided with a cathode by following the teachings of this invention; and

Figure 2 is a longitudinal section of an embodiment of the apparatus of this invention with a tube being shown in position during the cathode processing step.

`@referring to the drawings, and more particularly to Figure l, an evacuated image tube generally indicated by the reference numeral 1 is comprised of a cylindrically shaped glass envelope 2 and opposite transparent end plates 3 and 4 which serve as bases upon which the cathode and anode elements, respectively are formed. Metallic focusing, accelerating and the like electrodes enerally indicated by the reference number 5, which are secured in suitable positions inside the envelope 2, are arranged between the end plates 3 and 4 for directing an electron image emitted by the cathode surface 6 formed on the end plate 3 through the length of thetube onto the luminescent anode 7 formed on the end plate 4. The construction of this and similar image tubes and the operation thereof are well known to the art and need not be explained, further here. The invention is directed primarily to the formation of the radiant energy-sensitive cathode 6 on the inner surface of the tube face plate 3 as will now be more fully explained.

With reference to Figure 2, the apparatus constituting an embodiment of this invention is comprised of an outer support casing 8 of annular cross section and two coaxially arranged cylindrical enclosures 9 and 10 supported inside the casing 8. Suitable mounting screws 21 serve to mount securely the enclosure 9 with respect to the casing 8, and similar fastenings 11 serve in mounting the enclosure 10 in the upper end of the casing 8. The enclosure of chamber 10 is sealed on its topside by means of a plate or the like 12 While the lower end thereof is provided with a tapered bafe 13 which extends a short distance downwardly into the interior of the lower enclosure or chamber 9 and which is further provided with a tube-receiving opening 14. The space between the two chambers 9 and 10 and the casing 8 is filled with suitable heat-resistant packing or insulation, such as glass wool or the like. The lower end of the chamber 9 is closed by means of a removable plate 15 which is slotted to accommodate an exhaust tube or tubulation 16 which serves a purpose to be more fully explained hereafter.

rl`he arrangement thus far described is substantially completely enclosed, thereby reducing the tendency of air currents being developed inside Or between the two chambers 9 and 10.

eerdere The chamber 9 is provided with electrical radiant heating elements 17 of suitable design which are arranged around the periphery thereof and which in the preferred embodiment extend axially of the chamber. Similarly, suitable heating elements i8 are mounted on the inside walls of the chamber itil. For maintaining the temperatures of the respective heating elements t7 and 18 at the proper level, thermostats or temperaturecontrolling devices 19 and 20, respectively, are operatively coupled thereto. In general, the thermostat i9 is set to provide a higher temperature Within the chamber 9 than is produced in the chamber i@ under the control of the thermostat 2,0.

In operation, a tube completely assembled as illustrated in Figure 1 is supported in an upright position by means of the stii,C evacuating conduit iid, and the tube is completely evacuated by the use of suitable vacuum or exhausting pumping mechanism (not shown) operatively connected to the conduit. -At this stage of processing, the face plate 3 of the tube is completely transparent and free of dirt and other foreign material. As the tirst step in forming the cathode on the inner surface of this face plate 3, a silver film of minute substantially transparent thickness is evaporated on the face plate from a bead of silver exposed to the face plate and Supported by suitable heater wires in accordance with common practice. Next, a measured quantity of pure oxygen is admitted to the interior of the tube through the conduit i6 for oxiding the silver thus far deposited. In accordance with conventional practice, this oxidation is expedited and made more complete by producing a glow discharge in the oxygen by means of, for example, a radio frequency field positioned adjacent the face plate. Upon completion of this oxidation step, the excess oxygen is pumped out of the tube through the conduit 1.6. Another layer of silver is now evaporated onto the silver oxide lm which may, if desired, be oxidized in the same manner as explained in the foregoing.

At this state of processing the tube is now ready for applying the caesium onto the silver oxide base tilm. In' the present instance, a capsule containing caesium chromate and pure silicon is permanently mounted inside the tube and is heated by passing an electrical current therethrough. As av result of this heating, pure caesium is released and the silicon is chemically combined with the chromate to provide a stable compo-sition which does not interfere with the operating characteristics of the tube.

Before releasing the caesium, the oven of Figure 2 is dropped over the tube l to such a position as will place the face plate end of the tube inside the chamber iii and the remaining portion of the tube in chamber 9. It should be mentioned at this point, that it is desirable to keep the size of the tube-receiving opening 14 as small as possible to prevent the exchange of convection currents between the two chambers 9 and ld during the heating cycle.

With the oven so positioned, the thermostat 19 is adjusted to provide a temperature Within the chamber 9 which is of suicient magnitude to heat the envelope 2 to a temperature somewhat in excess of 200 degrees C. Radiant heat from the heating element 17 serves in heating the metal electrodes inside the tube to a suitable high temperature also in excess of 200 degrees C. At the same time, thermostat 2b is adjusted so that the heating elements 13 will radiate heat in sufficient quantity to maintain the temperature of the face plate 3 between 180 degrees C. and 200 degrees C.

With the differential in temperatures, the caesium released inside the tube will tend to deposit or condense on the cooler tube portions, and further will be driven from the hotter portions of the tube and thereby be prevented from depositing or reacting therewith. Also, the caesium by reason of its atiinity for oxygen tends to iiow toward the silver oxide iiim on the face place and upon contact therewith tends to react with the oxygen molecules in the silver oxide. rthe resulting chemical reaction produces a film having a composition of silver-oxidecaesium, the exact constituency of 'the composition being common and not important to this invention. Silveroxide-caesium cathodes have been known for many years and vary in the constituency thereof depending upon design requirements.

Since the two chambers, 9 and 116, are completely eniosed, with exception of the communicating opening le, convection air currents are kept at a minimum, thereby fac. ating the control of the face plate temperature within the critical limits explained previously. A differential heating of the tube is further made possible through the use of the radiant heat elements, 17 and 18, which are directly exposed to the tube. All sides and portions of the tube exposed to the respective heating elements are maintained at the proper temperatures for causing the caesium vapor to iow toward the face plate 3.

r'hermionic electron emission from tbc cathode during this differential heating and the rele ot cacsium is continuously measured in accordance with conventional practice to determine photoelectric sensitivity. The release of caesium is halted at or near tic point of peal: emission, at which time the temperature in the cathode oven is Suddenly increase to approximately 200 C. for not more than one minute as determined by a second peak in emission. The heating cycle is now completed and the tube is allowed to cool.

This sudden increase in cathode temperature has been found to speed up the cathode material reaction and to improve infra-red sensitivity. Also, more uniform sensitivity results in a production run of tubes.

The operating characteristics of the tube fabricated in accordance with the teachings of this invention are superior to those constructed in accordance with previous arrangements, since the operation of the prior tubes was obstructed in some degree by spurious electron emission from the caesium which reacted with the metallic ciectrodes and the internal surfaces of the tube envciope. In following the present invention, undesired sensitization of the tube walls and the metallic electrodes is kept at :t minimum, thereby providing for improved tube operation.

White there has b `en described what is at present considered the preferred embodiment of the invention, it will be obvious to those skiiled in the art that various changes and modiiications may be made therein without'. departing from the invention, and it is, therefore, aimed in thc appended claim to cover @Q1 such changes and modifications as fall within the truc spirit and scope or" the invention.

What is claimed is:

Apparatus for use in forming a photosensitive cathode on an end face plate of a tube, said apparatus comprising: an upper heating chamber having a closed upper end and having radiant heat generating elements disposed therein arranged to heat said upper chamber' to a first elevated temperature; a lower heating chamber having its upper end joined to the lower end of upper heating chamber and having an opening formed end for admitting said tube to the interior of said upper and lower chambers, said lower chamber having radiant heat generating eierients disposed therein arranged to heat sfr-.icl lower chamber to a second elevated tem-perature higher than sai-d first elevated temperature in. said upper chamber; and a baliie member separating said upper and tower chambers and having a central opening formed therethrough communicating with said upper and lower chambers and arranged to support said tube therein with said race plate disposed in said upper chamber and the remainder thereof disposed in said lower chamber.

(References on following page) References Cited n the lc of this patent 2,146,427 Hawkins Feb. 7, 1939 2,161,458 Boer et a1 June 6, 1939 UNITED STATES PATENTS 2,208,129 Ingram 1 July 16, 1.940 1,904,895 Campbell Apr. 18, 1933 2,264,496 Wollentin et a1. Dec. 2, 1941 2,102,760 Stilwell Dec. 21, 1937 5 2,413,442 Found Dec. 31, 1946 

